Impedance matching for a dual band power amplifier

ABSTRACT

An exciter matching circuit (125), interstage matching circuit (134), and harmonic filter matching circuit (140) match impedances at the input to a two-stage power amplifier (130), between the first stage (132) and the second stage (136) of the power amplifier (130), and at the output of the power amplifier (130) for more than one frequency band of interest. In a GSM/DCS dual band radiotelephone (101), the matching circuits (124, 134, 140) provide low return loss at 900 MHz when the dual band transmitter (110) is operating in the GSM mode. The harmonic filter matching circuit (140) also filters out signals at 1800 MHz, 2700 MHz, and high order harmonics. When the dual band transmitter (110) is in DCS mode, however, the matching circuits (124, 134, 140) provide a low return loss at 1800 MHz and filter out signals at 2700 MHz and harmonics of 1800 MHz.

FIELD OF THE INVENTION

This invention relates generally to dual band communication systems, andmore particularly to impedance matching circuits for a power amplifierin a dual band transmitter.

BACKGROUND OF THE INVENTION

A dual mode transmitter can operate using two different systems.

For example, an AM/FM dual mode transmitter can transmit both amplitudemodulated and frequency modulated signals. For radiotelephones, a dualband transmitter can operate using two different cellular telephonesystems. For example, a dual band GSM/DCS radiotelephone can use theGlobal System for Mobile Communications (GSM), which operates at 900MHz, and the Digital Communications System (DCS), which is similar toGSM except that it operates at 1800 MHz.

In any radiotelephone, the power amplifier at the final stage of thetransmitter should be matched to the impedance of the antenna.

Additionally, harmonics of the transmitted frequency band should besuppressed to reduce interference with other communication systemsoperating at the harmonic frequencies. With a GSM/DCS dual bandtransmitter, it is difficult to suppress the first (1800) MHz harmonicduring 900 MHz GSM transmissions and yet pass the 1800 MHz signal duringDCS transmissions. Also the output impedance of a radiotelephone poweramplifier should be matched to the antenna so that the impedance at theoutput of the amplifier is at the optimum impedance for power efficientamplification.

Thus, there is a need for a dual band power amplifier that can suppressharmonic frequencies during a first mode of transmission and alsoproperly pass signals during a second mode of transmission, even whenthe signals of the second transmission are at or near a harmonicfrequency of the first mode of transmission. There is also a need for adual mode power amplifier with a limited number of parts and a lowcurrent drain.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of a communication system having matchingcircuits according to a preferred embodiment.

FIG. 2 shows a diagram of the exciter matching circuit according to thepreferred embodiment.

FIG. 3 shows a diagram of the two-stage power amplifier according to thepreferred embodiment.

FIG. 4 shows a diagram of the harmonic filter matching circuit accordingto the preferred embodiment.

FIG. 5 shows a graph of a return loss signal and an attenuation signalat the output of the harmonic filter matching circuit in GSM modeaccording to the preferred embodiment.

FIG. 6 shows a graph of a return loss signal and an attenuation signalat the output of the harmonic filter matching circuit in DCS modeaccording to the preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Three matching circuits enable a modulator, power amplifier, and antennaof a radiotelephone transmitter to efficiently amplify and transmitsignals at more than one frequency band while suppressing first, second,and higher order harmonics. An exciter matching circuit matches theimpedance at the output of the modulator to the impedance at the inputof the power amplifier for both modes of a dual band transmitter. Aninterstage matching circuit has a switch to match impedances between afirst stage and a second stage of a power amplifier during differentbands of operation. Finally, a harmonic filter matching circuit uses aswitch to match impedances and adjust the filter pass band of a combinedfilter and matching circuit during different modes of operation.

FIG. 1 shows a diagram of a communication system 100 having matchingcircuits 125, 134, 140 according to a preferred embodiment. Thecommunication system 100 shown is a cellular communication system with ahandset radiotelephone 101 and a base station transceiver 190, however,a different communication system could be substituted, such as amodulator/demodulator (MODEM), a paging system, or a two-way radiosystem. The radiotelephone 101 is a dual band GSM/DCS radiotelephone,however, other transmission modes with constant envelope modulationschemes can be substituted for either the GSM mode, the DCS mode, orboth modes. Other constant envelope modulation communication systemsinclude Advanced Mobile Phone Service (AMPS) and ETACS (European TotalAccess Cellular System), which use frequency modulation (FM), andPersonal Communication System (PCS) 1900 which uses Gaussian MinimumShift Keying (GMSK) as does GSM and DCS. Transmission modes may also beadded to create a tri-mode or quad-mode radiotelephone.

The radiotelephone 101 includes a microphone 105 for picking up audiosignals. In a dual band transmitter 110, the audio signals are coded bya speech coder 115 and sent to a modulator 120. Depending on the mode inuse, the modulator 120 mixes the coded signals to 900 MHz in the case ofGSM or 1800 MHz in the case of DCS. An exciter matching circuit 125includes a bipolar junction transistor (BJT) and matches theapproximately 50 Ω impedance at the BJT output to the approximately 7 Ωimpedance at the power amplifier 130 input for the frequency bands ofinterest, which is either at 900 MHz or 1800 MHz depending on the modein use. Power amplifier 130 is preferably a gallium arsenide (GaAs)field-effect transistor (FET) two-stage amplifier with a first stage 132and a second stage 136. Other device types, however, such as siliconBJTs or silicon FETs could be substituted for the GaAs FETs. Between thetwo stages is an interstage matching circuit 134 that optimizes theimpedance matching at either 900 MHz or 1800 MHz depending on the modein use. At the output of the power amplifier 130, which has an impedanceof approximately 8-10 Ω and sometimes varies depending on thetransmitter mode in use, a harmonic filter matching circuit 140 matchesthe outgoing signal to the approximately 50 Ω antenna 155 at thefrequency band of interest and filters out first, second, and higherorder harmonics of the signal. The matched impedances presented to thepower amplifer input and the power amplifier output by the excitermatching circuit 125 and the harmonic filter matching circuit 140determine the efficiency of the power amplifier.

The transmitted signal is received by a complementary transceiver 190,such as a GSM cellular base station, through an antenna 195. A DCS basestation is also compatible with the GSM/DCS radiotelephone 101, andother transceivers would be compatible with PCS, AMPS, or ETACS dualmode radiotelephones. Signals from the base station transceiver 190 aretransmitted from the antenna 195 of the base station and received by theantenna 155 of the radiotelephone 101. A duplexer 150 in theradiotelephone 101 controls whether the antenna 155 is transmitting orreceiving signals. Received signals are sent through the duplexer 150 toreceiver 160. In the receiver 160, a radio frequency (RF) receiver 165prepares the signal for demodulation, a demodulator 170 demodulates thesignal, and a speech decoder 175 decodes the demodulated signal to anaudio format for reproduction on speaker 180.

FIG. 2 shows a diagram of the exciter matching circuit 125 according tothe preferred embodiment. When a GSM signal at 900 MHz emerges from themodulator 120 (shown in FIG. 1), certain components of the excitermatching circuit 125 dominate the impedance response to promote a matchto the power amplifier (shown in FIG. 1) at 900 MHz while rejectingother frequencies. Likewise, when a DCS signal at 1800 MHz comes fromthe modulator 120, different components dominate the impedance responseof the exciter matching circuit 125 to create a good match at 1800 MHzwhile creating a poor match at other frequencies.

The modulator 120 is isolated from the power amplifier 130 (shown inFIG. 1) using a resistance buffer with resistor 205 and resistor 207. A1 pF capacitance 215 is also connected from ground to the base of a BJT210. The BJT 210 is used to amplify and transform the impedance of amodulated signal before the signal enters the power amplifier 130 (shownin FIG. 1). The output of the BJT is at approximately 50 Ω. Aquarter-wave transmission line 220 is connected from the collector ofthe BJT 210 to a constant voltage source V_(B2). This transmission line220 acts as an inductor when the modulated signal is at 900 MHz and actsas an open circuit when the modulated signal is at 1800 MHz. A 68 pFcapacitance 225 is connected between the voltage source V_(B2) andground, and a resistor 227 is parallel to the transmission line 220. Theresistor 227 stabilizes the BJT by providing a resistive terminationwhen the transmission line 220 acts at an open circuit. A 4.7 pFcapacitance 230 is also connected to the collector of the BJT 210, whichfunctions as a direct current (DC) blocking element and as an impedancetransforming element at 900 MHz. Two transmission lines 240, 250 connectthe signal from the capacitance 230 to the output of the excitermatching circuit 125, which connects to the power amplifier 130 (shownin FIG. 1). Between the two transmission lines 240, 250 is a 1.5 pFcapacitance 245 to ground.

During operation, when a 900 MHz GSM modulated signal enters the inputto the exciter matching circuit 125, the inductance of the transmissionline 220 and capacitance 230 dominate the impedance of the excitermatching circuit 125 to create a good match at 900 MHz at approximately7 Ω input impedance of the power amplifier 130 (shown in FIG. 1). Theother elements in the exciter matching circuit 125 have a negligibleeffect on the impedance at the 1800 MHz frequency band. In other words,the inductance of the transmission line 220 and the capacitance 230 actas a high pass filter that also transforms lower frequency signals.

When an 1800 MHz DCS modulated signal enters the exciter matchingcircuit 125, the transmission line 220 is open and the inductance oftransmission lines 240, 250 and the capacitance 245 dominate theimpedance of the exciter matching circuit 125 to create a good match at1800 MHz to the approximately 7 Ω input impedance of the power amplifier130 (shown in FIG. 1). In this case, the transmission line 220 andcapacitance 230 have a negligible effect on the impedance at the 900 MHzfrequency band. The inductance of the transmission lines 240, 250 andthe capacitance 245 act as a low pass filter that also transforms higherfrequency signals.

FIG. 3 shows a diagram of the two-stage power amplifier 130 according tothe preferred embodiment. An interstage matching circuit 134 matches theimpedances between the first stage 132 and the second stage 136 of thetwo-stage power amplifier 130. The interstage matching circuit 134optimizes the impedances at 900 MHz or 1800 MHz depending on whichtransmission mode is in use.

Two metal semiconductor field-effect transistors (MESFETs) are used aspower amplifier stages 132, 136 in the power amplifier 130. Alternativesto the MESFETs include silicon BJTs, silicon MOSFETs, and heterojunctionbipolar transistors (HBTs). Between the two stages 132, 136 is a 15 pFcapacitance 325, and at the source of the first stage 132 is a small 3nH inductance 335 which is connected to a voltage source V_(B3). The twostages 132, 136, the inductance 335, and the capacitance 325 areintegrated onto a chip 310. Outside of the chip 310, a 2.7 pFcapacitance 340 is connected between the inductance 335 and the voltagesource V_(B3). A 1000 pF capacitance 350 is also connected to thevoltage source VB₃ with a diode 370 connected from the capacitance 350to ground. A 1.5 kΩ resistor 360 with an input node 365 is connectedbetween the capacitance 350 and the diode 370.

When a voltage source is connected to the input node 365, the diode 370turns on and the 1000 pF capacitance 350 dominates the impedance of theinterstage matching circuit 134. The capacitance values are calculatedso that 900 MHz GSM signals from the first stage 132 of the poweramplifier 130 are matched to the second stage of the power amplifier 130(shown in FIG. 1) when the input node 365 is connected to a 2.7 Vpositive voltage source. When a zero, negative, or floating voltagesource is connected to the input node 365, the 2.7 pF capacitance 340and the 3 nH inductance 335 and the capacitance 350 dominate theimpedance of the interstage matching circuit 134 which then matches 1800MHz DCS signals to the second stage 136 of the power amplifier 130(shown in FIG. 1). Thus, the voltage source applied to node 365 is aGSM/DCS mode selection voltage. Voltage is applied to node 365 when theradiotelephone 101 is in GSM mode, and voltage is not applied to node365 when the radiotelephone 101 is in DCS mode.

FIG. 4 shows a diagram of the harmonic filter matching circuit 140according to the preferred embodiment. The harmonic filter matchingcircuit 140 uses both impedance matching and low pass filtering to pass900 MHz signals and suppress 1800 MHz, 2700 MHz, 3600 MHz, and higherorder harmonics during GSM mode transmissions while passing 1800 MHzsignals and suppressing 2700 MHz signals and 3600 MHz and higher orderharmonics during DCS transmissions.

The output of the power amplifier 130 (shown in FIG. 1) is connectedthrough a first transmission line 410 to a voltage source V_(B4).

Transmission line 410 is preferably a half-wave transmission line at2700 MHz. A 100 pF capacitance 412 is also connected to the voltagesource V_(B4).

A set of transmission lines 420, 430, 440, 450 is connected in series tothe output of the power amplifier 130. At the ends of each transmissionline is a connection from an approximately 3 pF capacitance 422, 442,452, 482 through a diode 415, 425, 435, 445 to ground. The capacitanceof each diode 415, 425, 435, 445 when the diode is off adds a fixedparallel capacitance to the switched capacitances 422, 442, 452, 483. Anadditional 1.8 pF capacitance 432 is connected in parallel to the firstcapacitance 422 and diode 415 pair.

This structure can be described as a cascade of four low-pass matchingsections. The reactance of the first three sections, which includetransmission lines 420, 430, 440, are switchable using diodes 415, 425,435, 435. Between each capacitance and diode pair is a 1.5 kΩ resistor416, 426, 436, 446 connected to node 465, which controls the switchingof the first three sections. A 100 pF capacitance 434 connects the node465 and ground. Additional 1 pF or less capacitances 462, 472, 492, 414,424, provide attenuation for the 2700 MHz, 3600 MHz and high orderharmonics of the 900 MHz GSM and 1800 MHz DCS signals. The reactance ofthe final section, which includes transmission line 450, is fixed. Thisfinal section suppresses 3600 MHz harmonics generated by the diodes 415,425, 435, 435 when they are off.

When a 2.7 V positive voltage source is applied to node 465, diodes 415,425, 435, 445 turn on, and the approximately 3 pF capacitances 422, 442,452, 482 and the inherent inductance in the diodes 415, 425, 435, 445filter out 1800 MHz signals. Thus, the GSM/DCS mode selection voltageused for the interstage matching circuit 134 (shown in FIG. 3) can alsobe used to control the operation of the harmonic filter matching circuit140. Positive voltage is applied to node 465 when the radiotelephone 101is in GSM mode, and negative, zero, or floating voltage is applied tonode 465 when the radiotelephone 101 is in DCS mode. The operation ofthe harmonic filter matching circuit 140 provides impedance matching at900 MHz when the GSM mode is selected via node 465 with signalattenuation at the harmonic frequencies of 1800 MHz, 2700 MHz, and 3600MHz as well as other high order harmonic frequencies. When the DCS modeis selected, however, the harmonic filter matches at 1800 MHz andprovides signal attenuation starting at 2700 MHz as well as 3600 MHz andhigher order harmonics.

FIG. 5 shows a graph of a return loss signal 540 and an attenuationsignal 550 at the output of the harmonic filter matching circuit 140(shown in FIG. 1) in GSM mode according to the preferred embodiment. TheX-axis 510 of the graph measures frequency in MHz while the Y-axis 520of the graph measures attenuation in dB. The return loss signal 540 hasa significant lowering in return loss signal at 900 MHz, which indicatesa good impedance match at the 900 MHz GSM frequency band. Also, at 900MHz, the attenuation signal 550 is close to 0 dB, which passes the 900MHz signal at full power. Meanwhile, at 1800 MHz, 2700 MHz, and 3600MHz, the attenuation signal 550 lowers to dampen harmonics of the 900MHz signal.

FIG. 6 shows a graph of a return loss signal 640 and an attenuationsignal 650 at the output of the harmonic filter matching circuit 140(shown in FIG. 1) in DCS mode according to the preferred embodiment. TheX-axis 610 of the graph measures frequency in MHz while the Y-axis 620measures attenuation in dB. The return loss signal 640 has a significantlowering in return loss signal at 1800 MHz, which indicates a goodimpedance match at the 1800 MHz DCS frequency band. Also, at 1800 MHz,the attenuation signal 650 is close to 0 dB, which is very differentthan the attenuation signal characteristic for the harmonic filtermatching circuit when it is in the GSM mode. The attenuation signal 650still lowers at 2700 MHz and 3600 MHz to dampen harmonics of the 1800MHz signal.

Depending on the systems used in the dual mode radiotelephone 101,component values of the exciter matching circuit 125, the interstagematching circuit 134, and the harmonic filter matching circuit 140 canbe adjusted to match only at the frequency bands of interest. Also,transmission lines within the three matching circuits can be replacedwith inductances to reduce size or to promote fabrication onto anintegrated circuit.

The exciter matching circuit uses impedance characteristics to promotematching of the modulator output and the power amplifier input of a dualmode transmitter at more than one frequency band of interest. Thematching characteristics within the exciter matching circuit changedepending on the frequency band of the input signal. The interstagematching circuit 134 uses a switch to add components, which varies thematching characteristic of the interstage matching circuit between thefirst stage and the second stage of a two-stage power amplifierdepending on the mode used by the dual mode transmitter. The harmonicfilter matching circuit 140 also uses switches to add components to varythe matching characteristic and the filter characteristic of theharmonic filter matching circuit between the output of the poweramplifier and the input of the antenna depending on the mode used by thedual mode transmitter.

Thus, the three matching circuits use very few additional components toprovide matching at more than one frequency band of interest and filterout undesired harmonics for dual mode transmitters dependent upon themode in use. While specific components and functions of the impedancematching for a dual band power amplifier are described above, fewer oradditional functions could be employed by one skilled in the art withinthe true spirit and scope of the present invention. The invention shouldbe limited only by the appended claims.

We claim:
 1. A multi-band transmitter having a power amplifier withpower amplifier input impedances and power amplifier output impedancesfor power efficient operation in a saturated mode at either a firstfrequency band or a second frequency band comprising:an exciter matchingcircuit, coupled to an input of the power amplifier, for selectivelymatching an output impedance of an exciter matching circuit in eitherthe first frequency band or the second frequency band to the poweramplifier input impedances, having:a first transmission line, coupledbetween a voltage source and a first transistor output; a firstcapacitor, coupled between the voltage source and a second transmissionline; a second capacitor, coupled between the second transmission lineand ground; a third transmission line coupled between the secondtransmission line and the power amplifier,wherein the first transmissionline and the first capacitor transform frequencies in the firstfrequency band and pass frequencies in the second frequency band, andthe second transmission line, the second capacitor, and the thirdtransmission line transform frequencies in the second frequency band andpass frequencies in the first frequency band; and a harmonic filtermatching circuit, coupled to an output of the power amplifier, forselectively matching the power amplifier output impedances in either thefirst frequency band or the second frequency band to an antenna.
 2. Amulti-band transmitter according to claim 1 wherein the firsttransmission line is a quarter-wave transmission line at the secondfrequency band.
 3. A multi-band transmitter according to claim 1 furthercomprising:a resistive path parallel to the first transmission line; anda third capacitor coupled between the voltage source and ground.
 4. Amulti-band transmitter having a power amplifier with power amplifierinput impedances and power amplifier output impedances for powerefficient operation in a saturated mode at either a first frequency bandor a second frequency band comprising:an exciter matching circuit,coupled to an input of the power amplifier, for selectively matching anoutput impedance of an exciter matching circuit in either the firstfrequency band or the second frequency band to the power amplifier inputimpedances; and a harmonic filter matching circuit, coupled to an outputof the power amplifier, for selectively matching the power amplifieroutput impedances in either the first frequency band or the secondfrequency band to an antenna, having:a first switchable low-passmatching section, having a first inductance, a first capacitance, and afirst switch; a fixed low-pass matching section, having a fixedinductance and a fixed capacitance; and a second switchable low-passmatching section, coupled between the first switchable low-pass matchingsection and the fixed low-pass matching section, having a secondinductance, a second capacitance, and a second switch.
 5. A multi-bandtransmitter according to claim 4 wherein the second switch is a seconddiode coupled between the second capacitance and ground.
 6. A multi-bandtransmitter according to claim 4 wherein the first switch connects thefirst capacitance to ground when the first frequency band is selected,and wherein the first switch disconnects the first capacitance fromground when the second frequency band is selected.
 7. A multi-bandtransmitter according to claim 4 wherein the fixed low-pass matchingsection passes the first frequency band and the second frequency bandand attenuates harmonics of the second frequency band.